The advantage of using both hands instead of just the one handed with a no-thumb (or thumb-only-inserted-halfway) delivery is control. Watch the release of a truly good two-handed bowler in slow motion. Their throwing hand stays underneath the ball in a more balanced, natural position since the other hand is there for balance and grip. Right before the release point, this hand is removed, so the bowler actually only throws the ball with the one hand. This offers much greater accuracy than the one-handed no-thumb release in general. One of the best bowlers in my area has been using a two-handed release for years, and he's just about as accurate as the other top-level bowlers in the area who have more standard deliveries.